In recent years, there has been a growing demand for reduction in size and weight of electronic devices as typified by mobile phones and, to meet this demand, flexible circuit board substrates which are advantageous to reduction in size and weight of electronic devices are coming into wide use in the field of electronics. In particular, flexible circuit board substrates in which a polyimide resin is used as an insulating layer have been in wide use on account of their good heat resistance and chemical resistance. A higher degree of integration of the circuitry realized recently by miniaturization of electronic devices and high speed and reliability of information processing have in concert turned attention of the parties concerned to the question of how to dissipate heat generated in the devices.
It is considered effective to improve the thermal conductivity properties of electronic devices in order to improve the capability of dissipating heat generated in the devices and studies are conducted to develop technologies for incorporation of a heat conductive filler in an insulating layer which constitutes a wiring board substrate or the like. In this case, a filler of high thermal conductivity such as aluminum oxide, boron nitride, aluminum nitride, and silicon nitride is incorporated in a resin which forms the insulating layer.
In an attempt to obtain exceptionally high thermal conductivity by application of this kind of technology, a heat-dissipating sheet filled with a combination of a plate-like heat conductive filler and a spherical heat conductive filler is reported in patent document 1. That is, the aim here is to improve the heat-dissipating capability by distributing the plate-like heat conductive filler in multiple layers in the matrix resin and distributing the spherical heat conductive filler between the layers of the plate-like filler. However, the plate-like heat conductive filler tends to form agglomerates and some of agglomerates which are thicker than the heat-dissipating sheet may protrude from the sheet thereby worsening the surface condition of the insulating layer and adversely affecting the external appearance of the sheet.
A heat conductive polyimide film composite in which particles of a heat conductive filler are dispersed is described in patent document 2 as a material useful for a polyimide-metal laminate. However, the disclosure made in such a wide range as in patent document 2 is not sufficient to produce a material which satisfies various properties required for flexible board substrates. That is, the coefficient of linear expansion increases when a non-plate-like filler alone is used. Moreover, it is difficult to provide the film with high thermal conductivity by using a filler of low thermal conductivity such as aluminum oxide alone and increasing the content of filler to obtain high thermal conductivity markedly deteriorates the properties of the film.
Further, a heat conductive resin sheet filled with a mixture of a granular inorganic filler and a flat filler is described in patent document 3; however, the sheet is defective in that high thermal conductivity cannot be obtained when the sheet is thin because of the average particle diameter of the granular inorganic filler being 1 to 3.6 times that of the flat filler. Still further, a highly heat-dissipating composition using a mixed filler of scaly boron nitride and a granular metal oxide is described in patent document 4; however, the heat distortion temperature of the resin ranges from −30° C. to 130° C. and the composition is not suitable for use in wiring board substrates intended for high-temperature packaging of semiconductors and the like.